Get FCM Root Url from Environment variable

src/main/java/com/google/firebase/internal/FirebaseProcessEnvironment.java

@@ -43,4 +43,9 @@ public static void setenv(String name, String value) {
   public static void clearCache() {
     localCache.clear();
   }
+
+  public static void deleteEnv(String name) {
+    localCache.remove(name);
+    System.clearProperty(name);
+  }
 }

src/main/java/com/google/firebase/messaging/FirebaseMessagingClientImpl.java

@@ -47,6 +47,7 @@
 import com.google.firebase.internal.AbstractPlatformErrorHandler;
 import com.google.firebase.internal.ApiClientUtils;
 import com.google.firebase.internal.ErrorHandlingHttpClient;
+import com.google.firebase.internal.FirebaseProcessEnvironment;
 import com.google.firebase.internal.HttpRequestInfo;
 import com.google.firebase.internal.SdkUtils;
 import com.google.firebase.messaging.internal.MessagingServiceErrorResponse;
@@ -60,14 +61,18 @@
  */
 final class FirebaseMessagingClientImpl implements FirebaseMessagingClient {
 
-  private static final String FCM_URL = "https://fcm.googleapis.com/v1/projects/%s/messages:send";
+  private static final String FCM_ROOT_URL = "https://fcm.googleapis.com";
+  private static final String FCM_SEND_PATH = "v1/projects/%s/messages:send";
+  private static final String FCM_BATCH_PATH = "batch";
+  private static final String GOOGLE_FCM_HOST = "GOOGLE_FCM_HOST";
 
   private static final Map<String, String> COMMON_HEADERS =
       ImmutableMap.of(
           "X-GOOG-API-FORMAT-VERSION", "2",
           "X-Firebase-Client", "fire-admin-java/" + SdkUtils.getVersion());
 
   private final String fcmSendUrl;
+  private final String fcmBatchUrl;
   private final HttpRequestFactory requestFactory;
   private final HttpRequestFactory childRequestFactory;
   private final JsonFactory jsonFactory;
@@ -78,22 +83,35 @@ final class FirebaseMessagingClientImpl implements FirebaseMessagingClient {
 
   private FirebaseMessagingClientImpl(Builder builder) {
     checkArgument(!Strings.isNullOrEmpty(builder.projectId));
-    this.fcmSendUrl = String.format(FCM_URL, builder.projectId);
+    String fcmRootUrl = FirebaseProcessEnvironment.getenv(GOOGLE_FCM_HOST);
+    if (Strings.isNullOrEmpty(fcmRootUrl)) {
+      fcmRootUrl = FCM_ROOT_URL;
+    }
+    this.fcmSendUrl = String.format(String.format(
+                    "%s/%s", fcmRootUrl, FCM_SEND_PATH),
+            builder.projectId);
+    this.fcmBatchUrl = String.format("%s/%s", fcmRootUrl, FCM_BATCH_PATH);
     this.requestFactory = checkNotNull(builder.requestFactory);
     this.childRequestFactory = checkNotNull(builder.childRequestFactory);
     this.jsonFactory = checkNotNull(builder.jsonFactory);
     this.responseInterceptor = builder.responseInterceptor;
     this.errorHandler = new MessagingErrorHandler(this.jsonFactory);
     this.httpClient = new ErrorHandlingHttpClient<>(requestFactory, jsonFactory, errorHandler)
       .setInterceptor(responseInterceptor);
-    this.batchClient = new MessagingBatchClient(requestFactory.getTransport(), jsonFactory);
+    this.batchClient = new MessagingBatchClient(requestFactory.getTransport(),
+            jsonFactory, fcmRootUrl);
   }
 
   @VisibleForTesting
   String getFcmSendUrl() {
     return fcmSendUrl;
   }
 
+  @VisibleForTesting
+  String getFcmBatchUrl() {
+    return fcmBatchUrl;
+  }
+
   @VisibleForTesting
   HttpRequestFactory getRequestFactory() {
     return requestFactory;
@@ -139,7 +157,7 @@ private BatchResponse sendBatchRequest(
       return new BatchResponseImpl(callback.getResponses());
     } catch (HttpResponseException e) {
       OutgoingHttpRequest req = new OutgoingHttpRequest(
-          HttpMethods.POST, MessagingBatchClient.FCM_BATCH_URL);
+          HttpMethods.POST, fcmBatchUrl);
       IncomingHttpResponse resp = new IncomingHttpResponse(e, req);
       throw errorHandler.handleHttpResponseException(e, resp);
     } catch (IOException e) {
@@ -319,22 +337,17 @@ private MessagingServiceErrorResponse safeParse(String response) {
 
   private static class MessagingBatchClient extends AbstractGoogleJsonClient {
 
-    private static final String FCM_ROOT_URL = "https://fcm.googleapis.com";
-    private static final String FCM_BATCH_PATH = "batch";
-    private static final String FCM_BATCH_URL = String.format(
-        "%s/%s", FCM_ROOT_URL, FCM_BATCH_PATH);
-
-    MessagingBatchClient(HttpTransport transport, JsonFactory jsonFactory) {
-      super(new Builder(transport, jsonFactory));
+    MessagingBatchClient(HttpTransport transport, JsonFactory jsonFactory, String fcmRootUrl) {
+      super(new Builder(transport, jsonFactory, fcmRootUrl));
     }
 
     private MessagingBatchClient(Builder builder) {
       super(builder);
     }
 
     private static class Builder extends AbstractGoogleJsonClient.Builder {
-      Builder(HttpTransport transport, JsonFactory jsonFactory) {
-        super(transport, jsonFactory, FCM_ROOT_URL, "", null, false);
+      Builder(HttpTransport transport, JsonFactory jsonFactory, String fcmRootUrl) {
+        super(transport, jsonFactory, fcmRootUrl, "", null, false);
         setBatchPath(FCM_BATCH_PATH);
         setApplicationName("fire-admin-java");
       }

src/test/java/com/google/firebase/messaging/FirebaseMessagingClientImplTest.java

@@ -44,6 +44,7 @@
 import com.google.firebase.OutgoingHttpRequest;
 import com.google.firebase.auth.MockGoogleCredentials;
 import com.google.firebase.internal.ApiClientUtils;
+import com.google.firebase.internal.FirebaseProcessEnvironment;
 import com.google.firebase.internal.SdkUtils;
 import com.google.firebase.messaging.WebpushNotification.Action;
 import com.google.firebase.messaging.WebpushNotification.Direction;
@@ -548,6 +549,36 @@ public void testFromApp() throws IOException {
     }
   }
 
+  @Test
+  public void testFromAppWithFcmUrlFromEnv() throws IOException {
+    FirebaseProcessEnvironment.setenv("GOOGLE_FCM_HOST", "https://test-host.com");
+    FirebaseOptions options = FirebaseOptions.builder()
+        .setCredentials(new MockGoogleCredentials("test-token"))
+        .setProjectId("test-project")
+        .build();
+    FirebaseApp app = FirebaseApp.initializeApp(options);
+
+    try {
+      FirebaseMessagingClientImpl client = FirebaseMessagingClientImpl.fromApp(app);
+      String expectedFcmSendUrl = "https://test-host.com/v1/projects/test-project/messages:send";
+      assertEquals(expectedFcmSendUrl, client.getFcmSendUrl());
+      String expectedFcmBatchUrl = "https://test-host.com/batch";
+      assertEquals(expectedFcmBatchUrl, client.getFcmBatchUrl());
+      assertSame(options.getJsonFactory(), client.getJsonFactory());
+
+      HttpRequest request = client.getRequestFactory().buildGetRequest(
+          new GenericUrl("https://example.com"));
+      assertEquals("Bearer test-token", request.getHeaders().getAuthorization());
+
+      request = client.getChildRequestFactory().buildGetRequest(
+          new GenericUrl("https://example.com"));
+      assertNull(request.getHeaders().getAuthorization());
+    } finally {
+      FirebaseProcessEnvironment.deleteEnv("GOOGLE_FCM_HOST");
+      app.delete();
+    }
+  }
+
   private FirebaseMessagingClientImpl initMessagingClient(
       MockLowLevelHttpResponse mockResponse, HttpResponseInterceptor interceptor) {
     MockHttpTransport transport = new MockHttpTransport.Builder()